Optical sensors are devices that can be used to detect the change in an ambient environment and/or existence of analytes in the ambient environment. For example, if the sensor is formed with a biologically derived binding agent that has selective chemical or biological interaction with a certain analyte, a biosensor can be formed. Biosensors have attracted considerable attention recently due to their potential applications in medical diagnostics, explosives detection, determination of food quality, genetic screening and environmental monitoring.
To date, many optical sensors have been proposed and demonstrated. Long-period grating (LPG) and fiber Bragg grating (FBG) based on the evanescent wave are two major techniques that are widely used in fiber optical sensors. In these sensors, a periodic structure is written in the fiber structure, and a change of the analyte changes the optical field in the fiber structure. This results in a change in reflection and/or transmission spectrum of the periodically modulated fiber. Fiber optical sensors have attracted much attention recently due to their excellent characteristics such as compatibility with the standard optical fiber and simple fabrication process.
Fiber optic long period gratings are photonic devices usually made by UV induced refractive index modulation in the core of optical fiber. Light is coupled between the fundamental guiding mode and the co-directional cladding mode of the fiber at specific wavelengths. Because of the high loss due to scattering in the cladding mode propagation, the light coupled out of the core mode is lost before it is able to couple back to the guiding mode and thus leaves an absorption notch at the corresponding wavelengths (i.e., the mode coupling wavelength) in the transmission spectrum. The sensing mechanism arises from the dependence of the mode coupling wavelength on the effective index of the guiding mode and the cladding mode, as well as the period of the periodic modulation to the refractive index of the fiber core. One unique feature of LPGs is their high sensitivity to the ambient refractive index surrounding the cladding of the optical fiber. It is for this reason that LPGs have been widely accepted for fiber optic refractive index measurements. An important development of this sensing structure is the ability to deposit a layer of biorecognition material which can change its refractive index upon analyte binding. This method has been applied for chemical sensing and for sensitive detection of antibody-antigen reactions.
Enhancing sensitivity is a key issue in the development of optical sensors and is especially important when the change in the ambient environment or in volume (e.g. size and concentration) of analyte is small. A surface plasmon resonance (SPR) is usually used to enhance sensitivity of optical sensors.
SPR is an optical phenomenon caused by charge density oscillations at the interface between two media with dielectric constants of opposite sign, such as a metal and a dielectric such as glass for example. When light of an appropriate wavelength interacts with the dielectric-metal interface at a defined angle, called the resonance angle, there is a match of resonance between the energy of the light photons and the electrons at the metal surface. This resonance is experimentally observed as a sharp minimum in light reflectance when the angle of incidence is varied. Alternatively, SPR can also be generated by use of a fixed angle white light source with a broad bandwidth, using spectral detection. Any change in the refractive index of absorbed layers at the metal surface will affect the SPR coupling conditions and produce a shift in the resonance conditions.
In SPR fiber optical sensors, the cladding of the fiber is partially removed and a gold layer is deposited symmetrically around the exposed fiber core. Despite the successful demonstration of high sensitivity SPR sensors based on bulk and planar waveguide devices, high performance fiber-optic SPR sensors have not been reported mainly due to difficulties in device fabrication and operation. It is difficult to deposit a homogeneous coating and achieve good chemical functionality when using a round surface such as that of a fiber. Stable signals are also difficult to achieve since the SPR resonance condition is strongly dependent on the polarization state of the light propagating in the fiber, the polarization state being difficult to control within fibers. Furthermore, due to the critical SPR conditions, the wavelength used for sensing is usually shorter than 1 μm. Generally, low-cost high-performance tunable and/or broadband light sources required for fiber optic SPR biosensors are not commercially available.
Since only a small portion of a light field propagates as the evanescent wave along the fiber, LPG sensors usually have low sensitivity as compared with the SPR sensors. As a result, LPG biosensors are usually used to detect large molecules that provide larger refractive index change experienced by the evanescent wave.